1. Field of the Invention
The present invention relates to a optical fiber distortion measuring apparatus and optical fiber distortion measuring method which detect back scatter light generated by directing an optical pulse into an assessed optical fiber, and based on the results of this detection, obtain the amount of distortion in the assessed optical fiber.
2. Background Art
When a distortion is generated at a position in an optical fiber, the frequency distribution (spectra) of the Brillouin scattering light generated at that position when a light pulse is directed into the optical fiber is shifted by an amount proportional to the amount of distortion, when compared with the case in which a distortion is not present.
Optical fiber distortion measuring apparatuses which measure the amount of distortion in an optical fiber which is the subject of the measurement (an assessed optical fiber) using this principle were conventionally known.
FIG. 5 is a block diagram showing an example of a conventional optical fiber distortion measuring apparatus. This apparatus comprises a light source 1, an optical coupler 2, a light frequency conversion circuit 3, a light pulse output circuit 4, an optical coupler 5, a light receiving circuit 7, an amplifier circuit 8, an A/D conversion circuit 9, a signal processing unit 10, a curve approximating unit 11, a peak frequency detecting unit 12, a distortion amount calculating unit 13, and a display unit 14. The operation of the optical fiber distortion measuring apparatus having the structure described above will be explained.
(1) Measurement of the Time Change Waveform
The optical fiber distortion measuring apparatus shown in FIG. 5 obtains the time change waveform shown in FIG. 6 by directing an optical pulse from one end of the assessed optical fiber. In FIG. 6, the horizontal axis indicates time from the application of the light pulse. Here, the time from the application of the light pulse corresponds to the distance from the input end of the assessed optical fiber 6 to each position in the assessed optical fiber 6. Furthermore, the vertical axis shows the intensity of the Brillouin scattering light generated at each position.
The measurement operation of the time change waveform described above by the optical fiber distortion measuring apparatus shown in FIG. 5 will be explained. In FIG. 5, light source 1 generates a constant light (CW light) and a series of light pulses having a constant difference in light frequency from the constant light. The constant light generated by light source 1 is directed into the light receiving circuit 7 via optical coupler 2, while the series of light pulses generated by light source 1 are directed to the light frequency conversion circuit 3 via optical coupler 2.
Light frequency conversion circuit 3 conducts a frequency shift of the light frequency of the series of light pulses generated by light source 1, and converts them to a predetermined light frequency xcexd. Then, light pulse output circuit 4 outputs one light pulse from among the series of light pulses having a light frequency of xcexd, and the outputted light pulse is directed into the assessed optical fiber 6 via optical coupler 5.
When the optical pulse is directed into the assessed optical fiber 6, Brillouin scattering light is generated at each position in assessed optical fiber 6. The Brillouin scattering light generated at each position in assessed optical fiber 6 is successively directed into light receiving circuit 7 via optical coupler 5 while being delayed by an amount of time proportional to the distance from the input end of the assessed optical fiber 6 to each position.
Using the constant light (CW light) generated by light source 1, light receiving circuit 7 successively conducts the coherent detection of the Brillouin scattering light generated at each position in assessed optical fiber 6, and outputs electrical signals proportional to the intensity of each Brillouin scattering light.
Amplifier circuit 8 amplifies the electrical signal outputted by light receiving circuit 7, and A/D conversion circuit 9 conducts the A/D conversion of the electrical signals amplified by amplifier circuit 8.
Signal processing unit 10 first conducts signal processing such as noise removal, logarithmic conversion, and the like with respect to the electrical signal values which were A/D converted, and then conducts plotting such that the electrical signal values are correlated with the amount of time elapsed from the application of the light pulse (that is to say, the distance from the input end of the assessed optical fiber), and generates the time change waveform shown in FIG. 6. By means of the above processing, the Brillouin scatting light time change waveform is obtained in the case in which a light pulse having a light frequency xcexd is inputted.
(2) Calculation of the Amount of Distortion
Next, the method for calculating the amount of distortion of the assessed optical fiber 6 will be explained. When the amount of distortion of the assessed optical fiber 6 is calculated, the optical fiber distortion measurement apparatus shown in FIG. 5 repeats the operations described in (1) above while successively altering, by a specified value, the light frequency xcexd of the light pulse inputted into the assessed optical fiber 6, using the frequency conversion circuit 3. By means of this, the time change waveform, an example of which is shown in FIG. 6, is obtained with respect to a plurality of light frequencies.
FIG. 7 is a three-dimensional graph showing an example of time change waveforms relating to a plurality of light frequencies. In the figure, the horizontal axis indicates the light frequency xcexd of the light pulse inputted into the assessed optical fiber 6, while the vertical axis indicates the intensity of the Brillouin scattering light, and the axis which intersects both these axis at right angles (the angled axis) indicates the time from the input of the light pulse (the distance from the input end of the assessed optical fiber 6; that is to say, the position within the assessed optical fiber 6). In other words, the coordinate plane formed by the vertical axis and the angled axis in FIG. 7 corresponds to the coordinate plane shown in FIG. 6.
Furthermore, FIG. 8 is a graph in which the three-dimensional graph shown in FIG. 7 is sectioned at a certain distance D along the angled axis (the distance from the input end of the assessed optical fiber). In other words, FIG. 8 is a waveform (spectrum waveform) showing the frequency distribution (spectra) of the Brillouin scattering light at distance D.
When a spectrum waveform (see FIG. 8) is obtained at this certain distance D by means of this processing, then the curve approximating unit 11 shown in FIG. 5 applies the data shown by the spectrum waveform to a second-order formula, and produces an approximate curve (a second-order curve) of the spectrum waveform.
Then, the peak frequency determining unit 12 differentiates this approximate curve, and determines a light frequency indicating the maximum value of the intensity of the Brillouin scattering light (the peak frequency xcexdp).
Finally, distortion amount calculating unit 13 substitutes the peak frequency xcexdp determined by the peak frequency calculating unit 12 into the formula (1) shown below, and calculates the amount of distortion xcex5.
xcex5=(xcexdpxe2x88x92xcexdb)/(xcexdbxc3x97K)xe2x80x83xe2x80x83(1)
xcexdb: peak frequency when there is no distortion (characteristic value for assessed optical fiber 6)
K: distortion coefficient By means of the processing described above, the amount of distortion xcex5 at a position within the assessed optical fiber 6 (at a distance D from the input end) is determined and is displayed in display unit 14.
However, in the conventional optical fiber distortion measuring apparatus described above, when the amount of distortion of an assessed optical fiber is calculated, as described above, it is necessary to measure the time change waveforms of the Brillouin scattering light when a light pulse is applied with respect to a plurality (concretely, between 40 and 100) of light pulse frequencies while successively changing the light frequency xcexd.
Conventionally, 2-3 seconds were required to carry out the averaging processing in the measurement of a single time change waveform, so that when the distortion of an assessed optical fiber was measured, 40 to 100 times this amount of time, that is to say, maximally 6 minutes, were required.
In this way, in the conventional optical fiber distortion measuring apparatus, a very large amount of time was required for the measurement of the amount of distortion of the assessed optical fiber, so that the measurement efficiency was poor.
The object of the present invention is to provide an optical fiber distortion measuring apparatus and optical fiber distortion measuring method which make it possible to efficiently measure the amount of distortion of an assessed optical fiber in a short period of time.
The optical fiber distortion measuring apparatus of the present invention comprises:
a first measuring mechanism for measuring the difference in level between back scatter light from a undistorted assessed optical fiber and back scatter light from a distorted assessed optical fiber, with respect to the case in which the same light signal is directed into the undistorted assessed optical fiber, which is an optical fiber in a state in which no distortions are present, and the distorted assessed optical fiber, which is an optical fiber having the same structure as that of the undistorted assessed optical fiber, but in which distortions have been generated;
a second measuring mechanism for directing a light signal having a first light frequency into the undistorted assessed optical fiber and for obtaining an initial time change waveform indicating the intensity of back scatter light generated at each position in the undistorted assessed optical fiber;
a third measuring mechanism for directing a light signal having a first light frequency into the distorted assessed optical fiber and for obtaining a first time change waveform indicating the light intensity of the back scatter light generated at each position in the distorted assessed optical fiber;
a comparing mechanism for comparing the first time change waveform and the initial time change waveform and for detecting a detection point at which the light intensities differ from one another;
a fourth measuring mechanism for directing a light signal having a second light frequency into the distorted assessed optical fiber and obtaining a second time change waveform;
a correction mechanism for correcting a first light intensity, which is a light intensity at the detection point in the first time change waveform, on the basis of the difference in level, and for correcting a second light intensity, which is a light intensity at the detection point in the second time change waveform, on the basis of the difference in level;
a curve calculating mechanism for calculating, with respect to a spectrum waveform showing the relationship between the light frequency of the incident light and the intensity of the back scatter light generated at the detection points, a curve resulting from a parallel movement of a curve approximating the spectrum waveform relating to the undistorted assessed optical fiber, which curve satisfies the relationships between the first light frequency and first light intensity after correction, and satisfies the relationship between the second light frequency and second light intensity after correction;
a peak frequency calculating mechanism for calculating the light frequency exhibiting a maximal light intensity in the curve obtained by the curve calculating mechanism; and
a distortion amount calculating mechanism for calculating an amount of distortion of the assessed optical fiber at the detection point based on the light frequency obtained by the peak frequency calculating mechanism.
In accordance with the present invention, it is possible to measure the amount of distortion in an assessed optical fiber efficiently and in a short period of time.
The second, third, and fourth measuring mechanism may comprise, respectively, a light source for generating the light signal; a light frequency conversion mechanism for converting the frequency of the light signal generated by the light source, and directing the light signal into the assessed optical fiber; a light receiving mechanism for receiving back scatter light generated by the assessed optical fiber and for outputting an electrical signal which is proportionate to the intensity of the back scatter light, and a signal processing mechanism for outputting the time change waveform based on the electrical signal outputted by the light receiving mechanism.
The back scatter light handled by the first measuring mechanism may be Rayleigh scattering light, and back scatter light handled by the second, third, and fourth measuring mechanism may be Brillouin scattering light.
On the other hand, the optical fiber distortion measuring method of the present invention comprises:
a first process for measuring the difference in level between back scatter light from a undistorted assessed optical fiber and back scatter light from a distorted assessed optical fiber, with respect to the case in which the same light signal is directed into the undistorted assessed optical fiber, which is an optical fiber in a state in which no distortions are present, and the distorted assessed optical fiber, which is an optical fiber having the same structure as that of the undistorted assessed optical fiber, but in which distortions have been generated;
a second process for directing a light signal having a first light frequency into the undistorted assessed optical fiber and for obtaining an initial time change waveform indicating the intensity of back scatter light generated at each position in the undistorted assessed optical fiber;
a third process for directing a light signal having a first light frequency into the distorted assessed optical fiber and for obtaining a first time change waveform indicating the light intensity of the back scatter light generated at each position in the distorted assessed optical fiber;
a fourth process for comparing the first time change waveform and the initial time change waveform and for detecting a detection point at which the light intensities differ from one another;
a fifth process for directing a light signal having a second light frequency into the distorted assessed optical fiber and obtaining a second time change waveform;
a sixth process for correcting a first light intensity, which is a light intensity at the detection point in the first time change waveform, on the basis of the difference in level, and for correcting a second light intensity, which is a light intensity at the detection point in the second time change waveform, on the basis of the difference in level;
a seventh process for calculating, with respect to a spectrum waveform showing the relationship between the light frequency of the incident light and the intensity of the back scatter light generated at the detection points, a curve resulting from a parallel movement of a curve approximating the spectrum waveform relating to the undistorted assessed optical fiber, which curve satisfies the relationships between the first light frequency and first light intensity after correction, and satisfies the relationship between the second light frequency and second light intensity after correction;
an eighth process for calculating the light frequency exhibiting a maximal light intensity in the curve obtained in the seventh process; and
a ninth process for calculating an amount of distortion of the assessed optical fiber at the detection point based on the light frequency exhibiting a maximal value obtained in the eighth process.
The second, third, and fifth processes may comprise, respectively, a process for generating the light signal; a process for converting the frequency of the light signal, and directing the light signal into the assessed optical fiber; a process for receiving back scatter light generated by the assessed optical fiber and for outputting an electrical signal which is proportionate to the intensity of the back scatter light, and a process for outputting the time change waveform based on the electrical signal.
The back scatter light in the first process may be Rayleigh scattering light, and the back scatter light in the second, third, and seventh processes may be Brillouin scattering light.